The purpose of the proposed work is to define the structural-functional relationships involved in lymphokine activity. Probes used include anti-lymphokine antisera, reduction-alkylation procedures and dissociation techniques as well as conventional physico-chemical characterization procedures. We have found that human MIF consists of noncovalently linked subunits. In contrast, guinea pig MIF appears to be composed of subunits linked by disulfide bonds. Current studies are designed to determine which subunit is responsible for target cell binding and which contains the active site necessary for target cell activation (if, in fact, these sites are distinct). Antisera to MIF already available, as well as those in preparation, will be useful in answering these questions as well as in studies of in vivo lymphokine activity.